Process for carbonizing wood residues and producing activated carbon

ABSTRACT

An apparatus and process for preparing activated carbon from a wood or wood residue feedstock using fluidized bed technology. The fluidized bed apparatus has a number of wood residue inlets that allow differing residence times in the fluidized bed apparatus appropriate for the wood or wood residue feedstock to be carbonized. The carbonized material may then be activated to form activated carbon.

This application is a continuation of U.S. application Ser. No.09/959,608, filed Feb. 4, 2002, now U.S. Pat. No. 6,808,390 which isbased on PCT Application No. PCT/AU00/00410, filed May 4, 2000, whichclaims priority to Australian Application No. PQ 0159, filed May 4, 1999and Australian Application No. PQ 0679, filed Jun. 1, 1999.

TECHNICAL FIELD

The present invention relates to a fluidized bed apparatus and a processfor carbonizing wood and/or wood residues, especially in a fluidized bedapparatus. The invention also relates to a process and an apparatus forthe production of activated carbon.

BACKGROUND TO THE INVENTION

The timber industry generates considerable quantities of wood residues.Bark is produced in the debarking of the logs. Sawing of the debarkedlogs produces slabs, edgings and sawdust. If the solid slab and edgingsare chipped for export pulp chips, the chips must be screened and thisproduces additional residues in the form of undersized and oversizedwoodchips. The timber is often seasoned then dressed and docked tolength before despatching to the markets. This produces dockings andplaner shavings residues. In many circumstances, there is poorutilization of the wood residues. Some of the bark can fetch a market asa garden mulch if it is sized. Sawdust, chip fines and planer shavingsare used by the mill for energy, in most cases, as a fuel for kilndrying timber and, in a small number of mills, for electricitygeneration. However, there are generally many more residues producedthan the energy requirements of the sawmilling industry. Sawdust is alsoused for composting into garden potting mixes. Generally, there is alarge surplus of residues as the markets for horticulture arediminishing as demand for the dark P.radiata bark has fallen as thelighter wood fibre product has gained in popularity. The use of chippedprunings is also increasing as Councils and householders are turning torecycling and self-sufficiency. Where the residues are not utilised,they must be disposed of by incineration and land-fill dumpimg. Thedisposal of wood residues not only puts a severe cost on the sawmillingoperation, particularly with the pollution restrictions imposed on airquality, water effluent into water-ways and ground water and thediminishing land-fill availability, but it represents a loss of apotentially valuable wood resource. Unlike coal which can be left in theground to mine at a later date, wood residues cannot be stored on a longterm basis and need to be processed when produced. Wood residues arealso bulky, as in the case of sawdust and shavings, and can also containa considerable quantity of water. Processing or utilization in-situoffers the advantage of avoiding the high cost of transportation.

When wood is heated, it loses free and hygroscopic water after which itwill carbonize at temperatures in excess of 270° C. Gas and vapours areproduced during carbonisation which, at some stage, becomes exothermic.There are many complex reactions occurring at the same time in thethermal decomposition of the various chemical components of wood.Practical carbonizing temperatures are in the range of 400–700° C. inorder to produce charcoal with low-volatile content without excessiveshortening of equipment life.

The volatile products consist of combustible gases and vapors. Theenergy value of the volatile products represents some 50% of the grosscalorific value of the original dry wood. Although there are significantproportions of valuable chemical compounds present in the volatileproducts, production on a larger scale is required to economicallyjustify the fractionation and recovery of these compounds. However, thetypical scale of operations in individual timber mills cannot produceeconomic quantities of volatile products. This material can presentproblems in handling due to its acidic, corrosive nature and it would bea serious pollutant if discharged into the environment. One way ofhandling the volatile products is to burn them as they are producedbefore they are able to condense. The waste heat can be recovered tosupply the energy requirements of the industry, hence optimizing thethermal efficiency of the carbonization process.

A viable system for the sawmilling industry would perform the threefoldpurpose of disposal of the wood residues, supply the energy requirementsof the milling and seasoning operations; and upgrade the excess materialinto a product which can provide a profitable return.

The applicant's earlier Australian Patent No. 547130 described a processthat achieved the above aims. This earlier patent described a processfor carbonizing wood by feeding wood into a fluidized sand bed preheatedto a temperature above the carbonizing temperature. The fluidized bedwas fluidized with a gas mixture that included an oxidizing gas. Thereaction conditions within the bed was selected such that all or a majorproportion of the volatile components of the wood were burnt duringcarbonization, either as the volatiles were produced in the bed orpartially burnt in the bed and the remainder in an afterburner. Theburning of the volatile components provided sufficient energy to supplythe heat required by the process as well as provide an excess of heat.Charcoal produced by the process was recovered as product.

The process described in Australian Patent No. 547130 provides asatisfactory process for treating timber milling residues to obtain avalue-added product.

The present inventors have now developed an improved process forcarbonizing wood, such as timber milling residues.

Activated carbon is an amorphous form of carbon having a very highspecific surface area. Activated carbon has high absorptivity for alarge number of substances and is widely used as an adsorbent in manyindustries, including water treatment, sugar refining, gold mining,brewing, gas adsorption and air conditioning, to name but a few.Activated carbon may be obtained by the destructive distillation ofwood, nut shells, animal bones or other carbonaceous materials. It isalso possible to produce activated carbon by activating a carbonfeedstock, such as charcoal. Activation occurs by heating the materialto be activated to an elevated temperature, such as 800–900° C. withsteam or carbon dioxide to produce a carbon material having highporosity and a specific surface area that may be in excess of 1000 m²/g.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a processfor carbonizing wood residues to produce charcoal, said wood residuesincluding wood or woody-type particles of varying size and/or moisturecontent, the process including the steps of feeding the wood residue toa fluidized bed having a plurality of wood residue inlets, the wood orwoody-type particles of varying size and/or dryness being fed todiffering ones of the plurality of wood residue inlets according to anexpected time for carbonization for said wood or woody-type particles,the fluidized bed including a bed of inert particulate materialfluidized with or having injected therein a gas or gas mixturecontaining an oxidizing gas, carbonizing the wood residues in thefluidized bed under reaction conditions selected such that volatilecomponents in the wood residues are removed during carbonization and areburned in or above the bed or in an afterburner to thereby supply theheat requirements for carbonization and separating charcoal from theinert particulate material.

Preferably, the residence time of a wood or woody-type particles in thefluidized bed is largely determined by the wood residue inlet throughwhich the wood or woody-type particle is fed to the bed. In this manner,different residence times in the bed for different particles may beobtained by feeding the particles through different wood residue inlets.

The process may preferably further include grading the wood orwoody-type particles into a plurality of grades according to expectedtime for carbonization and feeding different grades to different of theplurality of wood residue inlets. The plurality of grades may includegrades based upon particle size, moisture content, wood species and typeof residue, for example, wood or bark. The wood or woody-type particlesmay include sawdust, planer shavings, shredded dockings, woodchips,bark, barkchips and larger wood such as blockwood.

According to a second aspect the present invention provides a processfor production of activated carbon from a carbon feedstock including thesteps of providing a fluidised bed reactor, adding particulate materialto the fluidised bed reactor to form a bed of particulate material,adding the carbon feedstock to the fluidised bed reactor, fluidising thebed and activating the carbon feedstock to produce activated carbon andrecovering activated carbon from the bed.

Preferably the carbon feedstock is the product of the first aspect ofthe present invention.

Preferably, the step of activating the carbon feedstock includes addingsteam to the fluidised bed. More preferably, steam is used as thefluidising gas in the fluidised bed. The steam is most preferablysuperheated steam. Activating gases other than steam may be used.Another suitable activating gas is carbon dioxide.

According to a third aspect the present invention provides an apparatusfor carbonizing wood residues to produce charcoal including a fluidizedbed reactor having a plurality of wood residue inlets for supplying woodresidues thereto, a discharge outlet for removing fluidized bed contentsfrom the fluidized bed and at least one fluidizing gas inlet,characterized in that residence time of a wood or woody type particle inthe fluidized bed reactor is dependent upon the inlet through which thewood or woody type particle is fed to the fluidized bed reactor.

The apparatus may further include separating means for separatingcharcoal from the fluidized bed contents removed from the dischargeoutlet and return means for returning inert particulate material to thefluidized bed reactor. The return means may comprise a conduit connectedto an inlet. This inlet may be one or more of tire wood residue inlets.The conduit may be provided with particulate material transport means,which may be a conveyor, a screw conveyor, an augur, pneumatic conveyingmeans or gaseous conveying means.

The separating means may comprise a screen means, especially a vibratoryscreen means.

The fluidized bed reactor should also include an exhaust gas outlet forremoving fluidizing gas and combustion gas from the fluidized bedreactor. A gas-solid separation means may be provided to separateelutriated solids from the exhaust gas. The gas-solid separation meansmay be a cyclone or an electrostatic precipitator.

The apparatus may further include an afterburner for burning anyuncombusted volatiles in the exhaust gas. A pre-heater for heating thefluidized bed reactor may also be provided to pre-heat the fluidized bedat start-up or during low temperature operation.

The apparatus may also include heat recovery means for recovering heatfrom the fluidized bed in the reactor. Heat recovery means may also beprovided for recovering heat from the exhaust gas and/or theafterburner.

According to a fourth aspect the present invention provides an apparatusfor producing activated carbon including a furnace, a reactor positionedinside the furnace, the reactor including solids inlet means forsupplying solids to the reactor and gas inlet means for supplying gas tothe reactor, at least one pipe connected to the gas inlet means of thereactor, said at least one pipe having at least a portion of its lengthextending within the furnace whereby gas flowing through said at leastone pipe to the reactor is heated to an elevated temperature by thefurnace.

Preferably, the reactor is a fluidized bed reactor.

Preferably, that at least one pipe has a substantial portion of itslength extending within the furnace. Preferably, that at least one pipeis positioned within the furnace and external to the reactor.

Preferably, the apparatus further includes a gas manifold inside thefurnace, the gas manifold having an inlet and a plurality of outlets,each of the plurality of manifold outlets having respective ones of aplurality of pipes extending therefrom, the plurality of pipes beingconnected at their other ends to the gas inlet means of the reactor.

Preferably, the gas manifold is positioned external to the reactor.Preferably, the gas manifold extends substantially around the reactor.Preferably, the gas manifold is positioned at an upper part of thefurnace.

Preferably, the gas inlet means of the reactor is a fluidizing gas inletmeans of a fluidized bed reactor. Preferably, the gas inlet means of thereactor comprises a plurality of gas ports. The gas ports are connectedto respective ones of the plurality of pipes.

The furnace may include at least one burner.

The reactor may include exhaust gas removal means for removing exhaustgas therefrom. The exhaust gas removal means may include solids removalmeans for removing solids from the exhaust gas. The solids removal meansmay comprise a screen, a filter or a cyclone. The solids removal meansmay return removed solids to the reactor.

The reactor may also include solids outlet means for removing solidsfrom the reactor. However, the reactor may be configured such thatsolids removal may take place via the solids inlet means. It may also beconfigured such that solids removal may take place by elutriationthrough the exhaust gas outlet.

In use of apparatus of the present invention is used to produceactivated carbon, calcined alumina and feedstock carbon are supplied tothe reactor. As the reactor is inside the furnace, the furnace heats thecontents of the reactor to the desired activation temperature.Fluidizing gas, in this case steam, is supplied to the gas inlet of thereactor via at least one pipe extending within the furnace. As the steamflows through the at least one pipe it is superheated to the desiredtemperature by the furnace. As the steam enters the gas inlet of thereactor, it is at the desired temperature and activation of the carbonfeedstock proceeds.

Although the apparatus of the present invention has been described asbeing used for production of activated carbon, it will be appreciatedthat the apparatus may be used in any process where it is desired topass a gas at elevated temperature through a bed of solids material.

DETAILED DESCRIPTION OF THE INVENTION

The inert particulate material may comprise any suitable particulatematerial that is able to be fluidized and does not undergo substantialreaction in the operating conditions experienced in the fluidized bed.Sand is a suitable particulate material for use in the presentinvention. Fine sand is preferred over coarse sand because fine sandgives better heat transfer to submerged surfaces (such as a heatexchanger within the bed) and to cold feed particles of wood residuesand promotes better combustion of volatile gases within the bed. Finesand also causes less attrition of submerged surfaces and products thencoarse sand. Ilmenite sand has been found to be particularly suitablebecause it has a fine size (typically below 1 mm), is very dense, wearresistant and has a high melting point. Thus, it can be used over a widerange of operating conditions. Ilmenite sand is a naturally occurring,relatively abundant, low cost material.

Calcined alumina may also be used as a suitable particulate material inthe present invention.

The reaction conditions used in the fluidized bed are those that aresuitable for carbonizing wood residues. The temperature may range from250° C. to about 1000° C., preferably 250° C. to 700° C., morepreferably with a temperature range of 400–600° C., most preferably atabout 500° C., being suitable for optimum charcoal production.

The present invention is based on the rapid drying and pyrolysis ofresidue particles in a fluidized bed which has a high rate of heattransfer between hot particulate material and wood particles. Thevolatile products of carbonization produced within the fluidized bed arecombusted on encountering oxygen. The volatiles burn rapidly, producingheat while the charcoal formed burns more slowly. The charcoal productis recovered from the fluidized bed by separation and cooling before ithas time to burn to a significant extent.

By maintaining an excess of oxygen in the bed, charcoal production iscontrolled by the kinetics of the combustion of the charcoal andvolatiles. At lower bed temperatures, typically 400° C.–500° C., therate of combustion of charcoal is slow and recovery of charcoal can behigh. At high temperatures above 600° C., charcoal can burn rapidly andresult in complete combustion. If bed temperatures are too low, such as400° C. or lower, the rate of combustion of volatiles can become tooslow to maintain the heating of the bed and much of the volatiles canescape the bed without burning. When this occurs, supplementarypreheating of the bed may be necessary and if the preheat burners cannot burn the volatiles escaping from the bed, an afterburner may also berequired to eliminate the discharge of unburnt volatiles.

Heat can be recovered from the combusted off-gases by using a heatexchanger. It is also possible to recover heat from the bed with a heatexchanger submerged in the fluid bed. This has the advantage ofutilizing the high heat transfer coefficients within the bed. Inaddition, by removing the heat from the bed, it is possible to increasethe carbonizing capacity of the bed.

Another feature of the present invention is that the operatingconditions can be altered to meet varying requirements for eithercharcoal production or heat production only. This is an especiallyuseful feature where the carbonization process forms part of anintegrated saw mill. Such integrated saw mills typically have dryingkilns associated therewith for drying sawn timber.

It has been found that by operating a fluid bed fluidized with air atbed temperatures above 650° C., rapid combustion occurs for sawdust andwoodchips. Efficient, total combustion occurs above 700° C. Maximum heatrecovery can be derived from this system through heat exchangers withinthe bed and/or in the combustion off-gases. If the fluid bed temperaturewas reduced, the charcoal formed as an intermediate phase burns moreslowly and can be recovered as a byproduct. It was found that at a bedtemperature around 500° C., good recovery of charcoal can be achievedwith complete combustion of volatile products of carbonization withinthe fluidized bed. (Carbonization, pyrolysis, or thermal decompositionoccurs when wood is heated above 250° C. The hemicellulose component ofwood decomposes at the low temperature to produce a charred wood). Thebed temperature can be regulated by controlling the feedrate of woodfuel. Within the design capacity of the system, a high feedrate of fuelwill increase combustion temperature within the fluid bed and viceversa.

This system can be used as a boiler or process heat production unit witha high turndown ratio exceeding that of conventional combustion systemsby lowering the combustion temperature to a theoretical limit of 250° C.to produce a byproduct charred wood. In practice however, the bedtemperature should be around 500° C. but can be lowered if the volatilegases which escapes the bed is burnt in an afterburner, or secondarycombustion chamber and enough of this heat transferred to the fluidizedbed to sustain the carbonization temperature. At 500° C., the charcoalproduct has a higher fixed carbon content than low temperature chars.

The system becomes a very flexible system for sawmills which can use theprocess heat for kiln drying of its sawn products. During the heatupcycle of the kiln and charge, the heat demand is at its maximum. Underthis situation, the fluid bed unit can produce its maximum heat outputby operating at a high bed temperature for complete combustion and themaximum design feedrate of wood fuel. As the timber charge is kiln driedat mid cycle, the heat demand drops to a low level. Here, the fluid bedcan operate in its charcoal producing mode to reduce heat output andmaximize charcoal production. Wood residue production in typicalsawmills far exceeds its energy requirements. In energy terms, only 20%of the total residue production can provide all the energy requirements.Some of this excess energy can be converted into a saleable charcoalbyproduct. The system can therefore be tailored to process the residueproduction and provide the energy needs to sawmills.

Charcoal is one of the products of the present invention.

Product charcoal is separated from the inert particulate material in thefluidized bed and recovered. Product charcoal may be separated from theinert particulate material in two ways:

-   -   i) at least a portion of the contents of the fluidized bed are        removed from the bed and the charcoal separated from the inert        particulate material, for example, by sieving or screening. The        charcoal is recovered as product and the inert particulate        material is recycled back to the fluidized bed, preferably        whilst still hot. Preferably, at least a portion of bed contents        is continuously removed and the separated inert particulate        material is continuously recycled to the bed. This method of        separation is limited by the particle size of inert particulate        material as the screen size must be larger than the particle        size of the sand.    -   ii) charcoal fines may be separated and recovered by elutriation        from the bed. The upward velocity of the fluidizing gas and        combustion eases will lift out particles of a size that their        terminal velocity is less than the upward velocity of the gases.        The wood-derived charcoal particles have a low particle density.        To maximize the degree of charcoal fines elutriation, the inert        particulate material preferably has a high particle density.        Further-more, fluidization velocities are preferably below a        velocity that would carry off particles of the inert particle        material. With bed attrition, breakdown of the inert particulate        material could see the fines caused by such breakdown reporting        in the charcoal fines product.

It will be appreciated that the present invention also extends to coverany other suitable process for recovering charcoal from the fluidizedbed.

The fluidizing gas used in the present invention is preferably air. Thesupply of fluidizing air should preferably be such to ensure adequateexcess of oxygen for the combustion of volatiles within the bed. Themaximum flowrate that may be used is that which can just promote themixing of the inert particulate material and the wood residues.Operating below this regime may result in bed segregation occurring.

The fluidized bed is preferably able to be sectioned off so that thesuperficial velocity can be adjusted to optimize each section. Byturning off fluidizing gas to a particular section, the bed in thatsection will be slumped or immobilized.

The fluidized bed is preferably configured as a linear slide fluidizedbed with multiple feed injection points along the bed. In thisarrangement, the positioning of the multiple feed injection pointscorresponds to the required residence times to carbonize the grade ofwood residue passing through each of the wood residue inlets orinjection points. To make the unit shorter, a unit with internal wallsin the fluidized bed section to channel the material in a spiral fashionfrom inlet to outlet may be used.

If required to promote better mixing of wood residues and the inertparticulate material, one or more mechanical stirrers may be introduced.

The fluidized bed may have a single discharge point. The fluidized bedmay be provided with a recycled particulate material inlet for returningrecycled inert particulate material to the bed. Alternatively, therecycled inert particulate material may be returned to the bed via oneor more of the wood residue inlets.

In the second aspect of the present invention the carbon feedstock maybe charcoal, such as charcoal produced from saw milling and other woodresidues such as bark, wood chips and saw dust. The charcoal is suitablyproduced by the process described in the applicant's Australian PatentNo. 547,130. However, especially preferred is the charcoal as producedaccording to the process of the present invention. Other carbonfeedstocks may also be used. Other suitable carbon feedstocks includecoke and chars based on various carbonaceous sources such as biomass,nut shell, fruit seed, fruit kernel, animal, peat, brown coal, coal,anthracite, petroleum, natural gas or other organic compounds andsubstances. Straight carbonaceous materials of the abovementionedfeedstocks can also be used as the process could convert these to anintermediate char stage.

The temperature during the activation step is preferably within therange of 650–1000° C., more preferably within the range of 800–900° C.,most preferably within the range of 800–850° C.

The time required to complete the activation step preferably fallswithin the range of 10 minutes to 3 hours.

The particulate material is preferably calcined alumina although anyother particulate material that is stable at the temperaturesencountered during the process and does not deleteriously react with thefeedstock carbon or the activated carbon may also be used. Sand is anexample of another suitable particulate material that may be used in theprocess. The particulate material ideally should not break up or weardown during fluidisation not melt nor soften under the high temperatureprocess conditions.

A particle size analysis of a calcined alumina suitable for use in thepresent invention is as follows:

+212 microns less than 1% −212 + 106 microns 40–60% −106 + 45 microns53–60% −45 microns  0–5%

In cases where steam is used as the activating gas, one of the byproducts of the process is water gas. In such cases, the processpreferably further comprises recovering the water gas and using thewater gas to provide the heating or energy requirements of theactivating process. If there is a surplus of water gas, the surplus gascan be reticulated as a fuel gas and/or used to generate electricitythat may be utilised on-site or sold to the electricity supply grid.

Waste heat from the process may be used to generate steam to supply theprocess steam requirements.

The process of the present invention may be a continuous process or abatch process. If the process is to be used to produce activated carbonfrom charcoal obtained from wood residues, the process is preferablyoperated at the wood processing site (such as a saw mill or a woodchipping plant). As the size and quality of the carbon feedstock mayvary widely in this situation, a batch process is preferred in order toprovide greater flexibility during operation. For example, residencetimes are much easier to alter in a batch process because it is simply amatter of changing the batch time.

If steam activation is used and the by-product water gas is recoveredfor energy requirements, it is preferred that a plurality of fluidizedbed reactors are used in order to even out the production of water gasand to ensure that water gas is constantly available. For example, iftwo fluidized bed reactors are used, one may be activating charcoal andproducing water gas whilst the other is being emptied and re-charged.

As mentioned above, calcined alumina may be used as the particulatematerial in the bed. Calcined alumina has a relatively low density andsmall particle size and this enables a deeper bed to be used for a givenpressure drop, when compared to denser and coarser bed materials such assand. Alternatively, a lower bed weight can be obtained which allows alower strength reactor to be used. This may be an important factor toconsider because the reactor is operated at elevated temperatures.Moreover, calcined alumina is white in colour and provides a visualcontrast to the black-coloured activated carbon. This allows a visualinspection to determine if adequate separation of the product from theparticulate material is being obtained.

The product activated carbon may be recovered by separating it from theparticulate material by any suitable method known to the person skilledin the art. The activated carbon is suitably removed from the solidmaterial by sieving. In a continuous process, the activated carbon maybe recovered by periodically removing a portion of the solid materialfrom the bed and separating the actuated carbon therefrom.Alternatively, the activated carbon may be elutriated by the exhaust gasand recovered therefrom. In batch processes, the entire solids load ofthe batch may be removed from the fluidized bed reactor and theactivated carbon subsequently removed therefrom. The hot bed materialcan be re-used for the next carbon activation batch in the hot state toconserve heat energy.

The present invention also relates to an apparatus for producingactivated carbon.

A preferred embodiment of the present invention will now be describedwith reference to FIG. 1. It will be appreciated that the followingdescription is intended to be illustrative of an embodiment the presentinvention and should not be considered to limit the present invention tothe embodiment described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an apparatus according to one aspect ofthe present invention.

FIG. 2 is a schematic drawing of an apparatus according to a secondaspect of the present invention.

FIG. 1 an apparatus is shown in accordance with the present invention,which is used for carbonizing wood residues in accordance with theprocess of the present invention. The apparatus includes a fluidized bedreactor 10 which contains a fluidized bed, generally indicated byreference numeral 12. The fluidized bed 12 includes particulatematerial, in this case fine ilmenite sand. The fluidized bed arrangementshown in FIG. 1 is a linear slide fluidized bed in which the material inthe bed is gradually moved from an inlet end 12 a to an outlet end 12 b.

The fluidized bed reactor 10 includes an air distributor 13 locatedbelow the fluidized bed of sand 12. Ambient air 14 passes through an airpreheater 15 to air distributor 13 and thereafter the fluidizing airpasses upwardly through the fluidized bed reactor 10.

The fluidized bed reactor 10 is provided with a plurality of feedinjection points 16, 17, 18, 19. Feed injection points 16, 17, 18, 19are used for feeding wood residues of differing grades into thefluidized bed 12. Due to the arrangement of fluidized bed 12 as a linearslide fluidized bed, one factor controlling the residence time ofmaterial in the bed is the position at which the material is injectedinto the bed. In particular, the further from outlet end 12 b materialis injected, the longer the residence time of that material in the bed.Therefore, if it is desirable or necessary to treat a feed stock of woodresidues that comprises wood particles or woody particles of differingsize, moisture content and composition, such that the wood particles orwoody type particles require different times for complete carbonization,it is possible to grade the wood residues into different grades ofmaterial depending upon their carbonization times and inject thedifferent grades through the appropriate feed injection point. Forexample, it has been found that dry shavings are fully carbonized veryquickly. Sawdust and shredded dockings take slightly longer to carbonizethan dry shavings. Woodchips take longer again and bark chips require aneven longer time for carbonization to occur. Accordingly, the woodresidues could be graded or classified into shavings, sawdust andshredded dockings, woodchips and bark chips. The bark chips would be fedto the fluidized bed through feed injection point 16. The wood chipswould be fed to the fluidized bed through feed injection point 17.Sawdust and shredded dockings would be fed to the fluidized bed throughfeed injection point 18 and dry shavings will be fed to the fluidizedbed through feed injection point 19.

The fluidized bed reactor includes an outlet 20 at outlet end 12 b.During operation of the apparatus shown in FIG. 1, the contents of thefluidized bed are continuously removed through outlet 20. It will beappreciated that the fluidized bed contents removed through outlet 20comprise a mixture of sand and charcoal. The fluidized bed contents fallonto a vibrating screen 21 where oversize lump charcoal is separatedfrom the sand, which passes through the screen 21. The lump charcoalslides downwardly from the screen through product outlet 22 where it isrecovered. The sand that passes through screen 21 travels throughconduit 23 and upwardly along conduits 24 and 25 to a distribution valve26 that controls the recycle of sand to the fluidized bed. It will beappreciated that the sand that is recycled from screen 21 is preferablyreturned to the fluidized bed reactor whilst it is still hot. This hasobvious implications for the energy efficiency of the process. In orderto ensure that the sand passes upwardly through conduits 24 and 25,these conduits may be provided with suitable solid transport means, suchas a conveyor, an auger or pneumatic conveying apparatus. Distributionvalve 26 regulates the position at which the recycled sand is returnedto the fluidized bed. In particular, the recycled sand may be returnedto the fluidized bed through conduit 27, conduit 28 or conduit 29.

The fluidized bed reactor also includes preheaters 30, 31 and 32, whichmay each comprise a gas burner. Preheaters 30, 31, 32 act to preheat thesand in the fluidized bed to the desired operating temperature duringstartup. The preheaters 30, 31, 32 also act to provide supplementaryheating to the fluidized bed if only small quantities of wood residuesare being fed to the fluidized bed or if large quantities of heat arebeing extracted from the fluidized bed.

When wood residues are fed to the fluidized bed 12 through one or moreof multiple injection points 16, 17, 18, 19, the wood residues arerapidly dried and pyrolised. The fluidized sand bed 12 provides a highrate of heat transfer between the hot sand and the wood residueparticles. The volatile products of carbonization or pyrolysis producedwithin the fluidized bed are combusted on encountering oxygen from theair used to fluidize the bed. The volatiles burn rapidly, producing heatwhile the charcoal formed burns more slowly. The charcoal product isrecovered from the fluidized bed by separation and cooling before it hastime to significantly burn.

At the operating conditions used in fluidized bed reactor 10 for theproduction of charcoal, which conditions include an operatingtemperature of approximately 500° C., it has been found that sawdustparticles fed to the fluidized bed reactor 10 require up to 2 minutes tocarbonize, depending on species and moisture content. Wood chips requireup to 4 minutes. As drying requires a significant time, high moisturecontent feedstocks will take longer to carbonize. Dry shavings have beenfound to be fully carbonized within 1 minute as compared with 2 minutesfor green sawdust. Dry, shredded dockings took 2 minutes to fullycarbonize, compared with 4 minutes for wood chips from the same species.Bark has been found to require longer residence time to carbonize thanwood, typically requiring 6 minutes.

As mentioned earlier, the volatile components driven off the woodresidues during carbonization are burned in the fluidized bed reactor10. The exhaust gas leaves fluidized bed reactor 10 through exhaust gasoutlets 33, 34, 35. The exhaust gas comprises a mixture of fluidizinggas and combustion products. The exhaust gas may also include someuncombusted volatiles. Fine charcoal particles are elutriated with theexhaust gas, as are any very small particles of sand that have beenformed by attrition or abrasion in the fluidized bed. In order to removethe solid particles from the exhaust gas and to recover the finecharcoal particles therefrom as product, the exhaust gas passes througha cyclone 36. The fine solids material elutriated with the exhaust gasis separated from the exhaust gas inside claim 36 and the solids arerecovered at an underflow 37 from the cyclone. Cleaned gas 38 leaves thecyclone. The exhaust gas line may be provided with an afterburner 39 tofully combust any remaining volatile compounds in the exhaust gas. Aheat exchanger 40 is provided in the exhaust gas line to recover heatfrom the exhaust gas. The exhaust gas leaving heat exchanger 40thereafter passes through blower or fan 41 to stack or flue 42.

Significant quantities of heat are also generated by combustion ofvolatiles in the fluidized bed reactor 10. In order to recover thisheat, the fluidized bed reactor 10 is provided with a heat exchanger 43,which heat exchanger suitably includes heating coils immersed withinfluidized bed 12.

In the event that the fluidization of the gas does not providesufficient mixing, the fluidized bed 12 may also be provided withmechanical stirrers 44, 45.

The fluidized bed arrangement shown in FIG. 1 includes a channel forlarge sized wood residues, such as wood chips, where the fluidized bedtransports the wood residue feedstock from the feed entry points to thedischarge end during which time the material dries and carbonises. Thebed material then falls onto a screen, especially a vibrating screen, torecover course charcoal product from the hot sand. The sand is conveyedback to the feed end to effect the bed circulation. The length of thechannel section and circulation rate of the sand are designed accordingto the desired residence time of the wood residue particle. For a givenlength of channel section, the residence time can be controlled by thesand circulation rate.

A disengagement space 46 above the bed allows the particles thrownupwards by the gas bubble eruptions at the surface of the fluidized bed12 to fall back without entrainment in the exhaust gas.

The fluidized bed reactor 10 is conveniently operated under a slightnegative pressure. This can be achieved by an induced draft fan 41 atthe flue-gas discharge. Air locks (not shown) such as the rotary type,at the various feed entry points and product discharges maintain thenegative pressure.

Coarse residues such as bark and woodchips can be fed by dropping themonto the bed from above. Fine and more easily carbonized feed stock,such as sawdust and shavings, is fed into the bed by, for example, ascrew feeder. The entry points 18, 19, for these residues, should bebelow the bed to eliminate the carry over of the fine material in thegas stream, which would otherwise result in bypassing of the bed.

The heat exchange system described with reference to FIG. 1 is basedupon heat transfer oil circulation as an efficient means of extractingheat from the bed. The bed's temperature of around 500° C. is wellsuited to heat transfer oil which should give a long service life sincea suitable wall temperature inside the heat transfer tubes can be moreeasily achieved.

The ilmenite sand in the fluidized bed 12 acts as an inert medium forenabling a fluidization of the range of wood residue particle sizes. Thesand is also a reservoir for heat transfer in removing heat from thecombustion gases and it heats the feedstock particles to effect dryingof the wood residues and raises their temperature to that required forcarbonization. It also provides heat to the heat exchanger. The sandcirculation from the inlet to the outlet of the fluidized bed conveysthe feedstock through its various stages of heating and thermaldecomposition. The circulation rate therefore controls the extent of thedecomposition process to some degree. Circulation rate can be controlledby the rate of discharge of bed material onto the screen for a givenvolume of sand in the system and the rate of return of sand into the bed12. An adjustable height weir and/or a valve arrangement can regulatethis flow at the outlet. Sand make up may be required as sand is lostthrough carry over into the cyclone and with the coarse product. Lowsuperficial velocities and efficient screening act to minimize sandloss.

The present invention provides a very flexible method and apparatus forproducing charcoal by carbonization of wood residues. The use ofmultiple entry feed points for the wood residues allows a wide varietyof different residues to be carbonized in a single reactor unit withoutthe necessity of designing separate units for the different grades ofwood residues. The fluidized bed carbonization with heat recoverydescribed in the preferred embodiment of the present inventionintegrates well with sawmilling and kiln drying operations since it hasthe potential of processing the total wood residue production and toproduce heat for kiln drying. Excess energy in the residues is convertedinto a charcoal product which can be more economically transported tomarkets since the mass is reduced to a small fraction of the originaland the value of the charcoal product is significantly increased. Thefluidized bed system described herewith also has the flexibility ofbeing able to operate as a straight combustor where all the fuelfeedstock is burnt to produce heat. This is particularly useful insituations such as the startup of timber drying kilns where the heatdemand from the cold kiln is at its maximum. During mid-cycle heating,where demand for heat falls, charcoal production can resume on a scaledictated by demand.

Referring to FIG. 2, this shows a schematic side elevation, partly incross-section, of an apparatus, in accordance with the presentinvention. It will be appreciated that the apparatus shown in FIG. 2 isintended to be illustrative of one aspect of the present invention andthat the invention is not to be limited to the illustrated embodiment.

The apparatus 110 shown in FIG. 2 includes a reactor 111 positionedinside a furnace 112. The furnace 112 is supported by support legs 113,114. The furnace 112 has external walls that are insulated by insulatinglayer 115. The insulating layer is conventional and may comprise anysuitable furnace insulation known to those skilled in the art. Thefurnace 112 includes burner 116 which is positioned for tangentialentry. The furnace also includes a second tangential entry burnerpositioned 180° apart from burner 116, but not shown in FIG. 2. Thefurnace has an exhaust gas outlet 117 for removing exhaust gas from thefurnace. The exhaust gas outlet 117 is connected to an exhaust gasconduit 118.

Reactor 111 is positioned within furnace 112. The reactor 111 includes asolids inlet 122 through which solids may be admitted to the reactor.The solids inlet 122 is connected to a hopper 123 by solids conduit 124.Hopper 123 may include a valve (not shown) to control the flow of solidsto the reactor.

The reactor also includes a plurality of gas inlet means, only one ofwhich is shown at reference numeral 125. The gas inlet means comprises aplurality of ports formed in the lower part of the reactor 111. Theplurality of ports extends in a line substantially around thecircumference of the reactor 111. Gas pipe 126 has one end connected togas inlet port 125 and the other end connected to gas manifold 127. Gasmanifold 127 extends substantially around the upper part of the reactor.The gas manifold 127 has a plurality of outlets that are connected byrespective ones of the plurality of gas pipes to respective gas inletports. The gas manifold 127 includes an inlet 128 through which gas isadmitted to the manifold.

The reactor 111 shown in FIG. 2 is configured as a fluidised bed reactorand it includes a bottom portion 138 of reduced diameter and an upperportion 129 of increased diameter. Solids are loaded into the bottomportion 128 and fluidised by the fluidising gas entering the reactorthrough the plurality of gas inlet port 125.

The reactor further includes an exhaust gas outlet 130 connected to anexhaust gas conduit 131. Exhaust gas must flow through cyclone 132(shown in dotted outline) prior to leaving the exhaust gas outlet 130.The cyclone 132 acts to reduce the amount of solid material entrained inthe exhaust gas leaving the reactor 111. Solid particles removed fromthe exhaust gas by cyclone 132 are returned to the reactor. This acts toreduce the amount of solids material elutriated by the exhaust gasleaving the reactor. The exhaust gas may be processed by further gascleaning equipment (not shown) such as a baghouse or an electrostaticprecipitator, in order to remove any elutriated fines therefrom.

The lower end of the reactor includes a solids outlet 133. The solidsoutlet 133 includes a solids control flow valve 134. A conduit 135connects the solids outlet 133 to a receiving hopper 136.

Operation of the apparatus shown in FIG. 2 will now be described. In thefollowing description, the apparatus shown in FIG. 2 is used in a batchprocess for the production of activated carbon from a charcoalfeedstock. Although the following description will describe a batchprocess, it will be appreciated that the process could equally beoperated as a continuous process.

In the batch process for the production of activated carbon fromcharcoal feedstock, the solids to be charged to the reactor are fed intohopper 123 and subsequently discharged from hopper 123 via solidsconduit 24 through solids inlet 122 into the reactor 111. Solids chargedto the reactor 111 suitably comprise a mixture of calcined alumina andcharcoal. The feedstocks used in experiments to date have includedcalcined wood pellets, sawdust charcoal and coconut shell charcoal. Thecalcined wood carbon pellets were pellets 1.6 mm in diameter passing a 3mm aperture screen. The sawdust charcoal had the following sizeanalysis:

−6.83 mm −3.35 mm 2.36 mm −1.7 mm −1.4 mm −1.18 mm −1.0 mm −0.85 mm −0.6mm +6.83 mm +3.35 mm +2.36 mm  +1.7 mm +1.4 mm +1.18 mm  +1.0 mm +0.85mm  +0.6 mm +0.3 mm −0.3 mm 0% 1.3% 3.1% 8.0% 8.2% 8.7% 10.5% 6.9% 12.0%32.3% 8.9%

A size analysis of the coconut shell charcoal was not conducted.Calcined alumina of the following size analysis was also added:

+212 microns less than 1% −212 + 106 microns 40–60% −106 + 45 microns53–60% −45 microns  0–5%

Burner 116 is ignited in order to cause an increase in the temperatureinside the furnace, which will thereby elevate the temperature of thereactor and the contents of the reactor. Steam is supplied via a steamline (not shown) to the gas inlet 128 of the manifold 127. The steamflows into the manifold 127 and subsequently through the gas outlet inthe manifold in through pipes 126 and into gas inlet port 125 of thereactor. As the steam flows through the manifold 27 and the plurality ofgas pipes 126, both of which are positioned external to the reactor andwithin the furnace, the steam is superheated by the furnace to thedesired temperature, for example, to a temperature within the range of650–1000° C., more preferably to a temperature within the range of800–850° C.

The superheated steam fluidises the solids in the reactor and thecombination of steam and elevated temperature causes activation of thecharcoal in the fluidised bed to produce activated carbon.

The exhaust gas leaving the reactor via cyclone 132 and gas outlet 130includes water gas. The exhaust gas passes through conduit 131 to awater gas recovery unit where any water vapour is condensed therefrom.The gas is then recovered for distribution to parts of the plant thatrequire a fuel gas. A portion of the water gas is advantageouslysupplied as a feed gas to burner 116 to thereby provide for the energyrequirements of the furnace. Excess water gas may be fed to other partsof the site as a fuel gas or alternatively may be fed to a gas turbineto generate electricity for use on site or for sale to the electricitygrid.

Once the batch process has been operated for sufficient time to convertsubstantially all of the charcoal feedstock to activate a carbon, solidsvalve 134 is opened and the solids content of the reactor 111 fallthrough solids outlet 133 and solids conduit 135 into hopper 136. Afterallowing a suitable time for the solids to cool to an appropriatetemperature (which time may be reduced by the use of a coolant) thesolids in hopper 136 are sieved to separate the calcined alumina fromthe activated charcoal. The activated charcoal is a product and isrecovered whilst the calcined alumina is fed to the next batch.

If a coolant is used to cool the solids recovered from the reactor 111at the end of a batch, the coolant is suitably water and the waste heatextracted from the solids by the water is preferably used to generatesteam for feeding to the gas manifold 127. Alternatively, the hot chargecan be sieved directly through a screen located below solids valve 134to separate the coarse activated carbon from the fine, calcined alumina.The alumina is then returned to the reactor via hopper 123 and solidsconduit 124. Feedstock carbon is then charged via hopper 123 and solidsconduit 124 for the next batch activation cycle.

In order to ensure a continuity of supply of water gas for the process,it is preferred that the apparatus shown in FIG. 2 comprises one of aplurality of similar apparatus. For example, the apparatus shown in FIG.2 may comprise one of five similar apparatus operated in batch mode toproduce activated carbon from charcoal.

The apparatus shown in FIG. 2 and the process for producing activatedcarbon described herein may suitably form part of an integrated woodtreatment plant, such as a sawmill or a woodchipping mill. In such anintegrated plant, sawn logs are processed to produce lumber productssuch as sawn timber or wood chips. The wood residues and wood wastesremaining from that process such as sawdust, bark, thinnings and woodchips may be processed in an associated wood carbonisation plant (forexample, as is described in the applicant's Australian Patent No.547130) in order to produce charcoal. The charcoal may then be fed tothe apparatus shown in FIG. 2 for conversion into activated carbon. Theactivated carbon is a value added product produced from what wouldotherwise be a waste stream from the plant.

The activated carbon that is produced by the process of the presentinvention may be either granulated activated carbon or pelletisedactivated carbon. Granulated activated carbon may be made by activatingcharcoal feedstock of suitable size. Activated carbon pellets arepreferably produced by manufacturing pellets of the charcoal feedstockand subsequently activating those pellets.

Those skilled in the art will appreciate that the invention describedherein may be subject to variations and modifications other than thosespecifically described. It is to be appreciated that the presentinvention encompasses all such variations and modifications that fallwithin its spirit and scope.

1. A process for the production of activated carbon from a carbonizedfeedstock, comprising the steps of: providing a fluidized bed reactor;adding particulate material to the fluidized bed reactor to form a bedof particulate material in said fluidized bed reactor; adding carbonizedfeedstock to the fluidized bed reactor; further fluidizing the fluidizedbed reactor; and adding superheated steam to the fluidized bed reactorto activate the carbon in the carbonized feedstock to produce activatedcarbon.
 2. The process of claim 1, wherein said carbonized feedstock isthe product of a process for carbonizing wood residues to producecharcoal, said wood residues including wood or woody-type particles ofvarying size and/or moisture content, the process including the steps offeeding the wood residue to a fluidized bed having a plurality of woodresidue inlets, the wood or woody-type particles of varying size and/ordryness being fed to differing ones of the plurality of wood residueinlets according to an expected time for carbonization for said wood orwoody-type particles, the fluidized bed including a bed of inertparticulate material fluidized with or having injected therein a gas orgas mixture containing an oxidizing gas, carbonizing the wood residuesin the fluidized bed under reaction conditions selected such thatvolatile components in the wood residues are removed duringcarbonization and are burned in or above the bed or in an afterburner tothereby supply the heat requirements for carbonization and separatingcharcoal from the inert particulate material.
 3. A process of claim 1,further comprising the step of recovering the activated carbon from thefluidized reactor bed.